Araldo's trainer, MIke Moroney, described the death as a tragic accident.

Surgeons said that Admire Rakti had collapsed and died from a massive heart attack or internal bleed.

Crowds five deep had surrounded the hosing bay holding Admire Rakti. Stewards and race club officials had placed a green tarpaulin over the horse's body.

The Japanese entourage travelling with the horse, many of whom had flown into Australia in the last 24 hours hoping to see Japan win its second Melbourne Cup, were understandably devastated.

Admire Rakti weakened considerably and staggered to the line in last place despite being sent out as the top pick for the race. An autopsy will be conducted. The horse's jockey, Zac Purton, said: "I felt something was wrong unfortunately and I didn't push him out otherwise it might have happened earlier."

Racing Victoria chief steward Terry Bailey said: "Obviously there will have to be an autopsy done and the vets are with it now. He collapsed while they were unsaddling him back at the stalls.

"The autopsy will tell the story. It will be done at Werribee. That's the normal procedure for any horse. We'll see what that brings up and whether that's natural causes."

Admire Rakti raced on the speed under Zac Purton but was never a winning hope after yielding ground at the 600-metre mark.

He was attempting to become the first horse since Ethereal in 2001 to complete the Caulfield-Melbourne cups double.

It is the second straight year a horse has died following the Melbourne Cup after Verema broke down in the back straight in 2013.

About 30 protesters from the Coalition for the Protection of Racehorses had gathered around the main gate at Flemington Racecourse chanting "racing kills", said the group's spokesman Ward Young.

Mr Young said they were letting racegoers know that a horse had died "because last year the only people who knew about Verema dying in the Melbourne Cup were the people who bet on her".

"Racing does kills horses and we think a lot more needs to be done to make horse racing safer. These incidents are a lot more frequent than people know about."

Mr Young said Admire Rakti's death was another example of horses being overworked on the racetrack.

He said in the past year about 125 horses have died during or shortly after a race.

The Coalition for the Protection of Racehorses recently attracted the ire of the industry over an anti-racing billboard showing a dead horse, which appeared on one of Melbourne's busiest streets.

The 22-metre board appeared on CityLink near Footscray Road bearing the slogan "Is the party really worth it?" in early October.

The sign was slammed by Australian Racing Board chief executive Peter McGuaran as "a terrible distortion of the reality of thoroughbred racing", and branded "highly inappropriate and distasteful" by Racing Victoria.